1. Field of the Invention
The present invention relates to an image forming apparatus that transfers visible images form image carriers to a surface of an endless belt or to a recording member held on the surface of the endless bet.
2. Description of the Related Art
In a typical image forming apparatus, toner images of mutually different colors are first formed on respective image carriers and a color image is then created by transferring those toner images in a superimposed manner from the image carriers onto the surface of an endless belt. In some image forming apparatus the toner images are transferred onto a recording paper held on the surface of the belt instead of transferring them directly on the belt.
The belt is stretched over rollers so as to form a loop. One of the rollers functions as a drive roller and others function as driven rollers. A belt drive motor drives the drive roller so that the belt rotates at a constant speed. However, the diameter of the drive roller may change due to changes in the environmental temperature over time. If this happens, the belt does not rotate at the intended speed. This leads to occurrence of misregistration between the toner images of the colors (color misregistration).
Meanwhile, there has been conventionally known an image forming apparatus that endlessly moves a belt member at a predetermined target velocity by detecting a moving velocity of the belt member by a velocity detector and feeding back the result of detection to a drive speed of a belt drive motor (for example, see Japanese Patent Application Laid-open No. 2004-220006 and Japanese Patent No. 3965357). This configuration allows the endless movement of the belt member at the target speed even if the diameter of the drive roller is changed due to changes in the temperature.
The inventors of the present invention are doing research whereby it is possible to share the drive motor between one of a plurality of photosensitive elements and the belt member. This configuration leads to reduction in cost of the configuration in which the belt member is caused to endlessly move at a target speed in the above manner. More specifically, when there are four photosensitive elements corresponding to toner images of Y (yellow), C (cyan), M (magenta), and K (black), the drive motor is shared between the photosensitive element for K and the belt member. As an object for dual purpose, the photosensitive element for K is selected from among the four colors for some reasons as explained below. Namely, conventionally, in a print job in monochrome mode, it is general that wasteful energy consumption and occurrence of wear of components are reduced by driving only the photosensitive element for K and stopping the drive of the photosensitive elements for Y, C, and M. Even if the configuration is adopted, if the photosensitive element for K is selected as the photosensitive element that shares the drive motor with the belt member, the belt member can be driven irrespective of different modes.
However, if at least one of the photosensitive elements, which is not necessarily the photosensitive element for K, shares the drive motor with the belt member, a following problem arises. More specifically, for the purpose of endless movement of the belt member at the target velocity, if the drive speed of a shared drive motor is controlled based on the result of detecting the belt velocity, an angular velocity of the photosensitive element driven by the shared drive motor may differ from that of the other photosensitive elements depending on the diameter of the drive roller. Such a difference in linear velocity between the photosensitive elements causes misregistration between the toner image on the photosensitive element of the former and the toner images on the other photosensitive elements.